Optical fibers have proven to be capable of carrying large amounts of data over long distances. However, there is always a demand for more data-carrying capacity, since each increase in capacity is soon followed by new applications that use it. One of the limitations on the data-carrying capacity of an optical fiber is chromatic dispersion. Since an optical pulse inherently contains a range of optical frequencies, if the velocity of an optical signal through a fiber depends on its frequency, the pulse will become broader as it propagates through the fiber, eventually reaching a point at which one pulse starts to interfere with neighboring pulses.
Standard single mode optical fiber has a zero dispersion wavelength, i.e. a wavelength at which the velocity of the signal as a function of frequency has a stationary value, in the range 1300 to 1324 nm. Therefore, the effects of chromatic dispersion are kept to a minimum as long as the fiber is used at a wavelength in this range.
For a number of reasons, however, the 1300 nm wavelength band is not the preferred wavelength band in which to operate. The loss minimum for the silica-based fibers that are currently prevalent falls at a wavelength of about 1550 nm and the erbium-doped fiber amplifier also works at around this wavelength, so the preferred wavelength band is now the 1550 nm band, which extends from about 1535 nm to about 1565 nm. New erbium-doped fiber amplifiers can use the wavelength range of 1530 nm to 1610 nm.
Therefore, a considerable effort has been put into designing optical fibers in which the refractive index profile, combined with the dispersion characteristics of the material, combine to produce a zero dispersion wavelength in the 1550 nm band, preferably at approximately 1550 nm. This is known as “dispersion shifted fiber” (DSF). A considerable amount of DSF has been made and installed and is in use.
In a further effort to increase data carrying capacity, attention has turned to wavelength division multiplexing (WDM) which is a way of increasing capacity by using a number of different wavelengths within the wavelength band of choice, each wavelength providing a channel which carries data signals independently of the channels at other wavelengths. Thus, the data carrying capacity of a fiber can be multiplied several times. Naturally, it is required to have as many different wavelengths carrying data as possible, so it is preferred to employ many closely spaced wavelengths to maximize the capacity. This is known as dense wavelength division multiplexing (DWDM).
A problem arises, however, when WDM, and especially DWDM, is used in dispersion shifted fiber, owing to a phenomenon known as “four wave mixing” (4 WM) or sometimes “four photon mixing”. This is a non-linear effect, which arises when the optical intensity in the fiber is high, as it has to be, at least when the optical signal is introduced into the fiber, in order to achieve a satisfactory transmission distance. When there are optical signals at two closely spaced frequencies, ω1 and ω2, 4 WM produces signals having frequencies of 2ω1−ω2 and 2ω2−ω1, or, assuming that ω2>ω1, and setting Δω=ω2−ω1, it produces signal having frequencies of ω2+Δω and ω1−Δω. This provides a loss mechanism, reducing the power of the original signals at frequencies ω1 and ω2, but also, and more damagingly, it produces cross-talk between different frequencies, since if the frequencies are equally spaced, which they will be, to fit the maximum number of channels into the wavelength band, the signals produced from two channels by 4 WM will coincide in frequency with adjacent channels at frequencies above and below those of the original two. 4 WM also causes problems with high bit rate (e.g. 40 Gb/s) systems using high transmission power.
Normally, the magnitude of 4 WM is limited, and it is quite a small effect. This is because of chromatic dispersion, which causes the different signal wavelengths to have different velocities, thereby reducing its efficiency. However, if the dispersion is zero or close to zero, 4 WM effects can build up and become a very serious limitation on the use of WDM.
As well as 4 WM, there is another non-linear effect that degrades the performance of DWDM systems at wavelengths near the zero dispersion wavelength. This effect is known as Cross-Phase Modulation (XPM). Here the rising and falling edges of one signal slightly change the refractive index of the fiber, thereby distorting the other signals of DWDM system. This effect is again most efficient when different wavelengths move along the fiber at the same speed (as is the case at or near the zero dispersion wavelength), as the interaction between the signals is maximized.
In fact, the preferred fiber for use with DWDM is so-called “non-zero dispersion shifted fiber” which has a small, but non-zero, dispersion throughout the wavelength band, typically in the range 1.5–4 ps/nm-km (see, for example U.S. Pat. No. 5,587,830).
However, there is a large legacy of DSF installed, and it is possible to increase its data carrying capacity by using WDM, or by using high bit rates at high power, as long as the wavelengths used keep clear of the zero dispersion wavelength. To keep clear of the zero dispersion wavelength, of course, it is necessary to know what its value is.
It would be possible to actually measure the dispersion of a cable, end-to-end, for a number of different wavelengths in the band, and to find the zero dispersion wavelength in that way, but there is a problem with this approach. In the manufacture of DSF, fiber sections of several km in length from different production runs are spliced together to form one long cable. This has the advantage, from the point of view of DSF, that the overall dispersion of the cable is an average of the dispersions of the various sections, so variations in the zero dispersion wavelength between different production runs are averaged out and the final cable has a zero dispersion wavelength which is more closer to the design value than would be possible in any one production run. Therefore, the zero dispersion wavelength of the cable as a whole does not necessarily correspond to the zero dispersion wavelength of any particular section of it.
To be able to use WDM in a DSF cable, or generally to avoid 4 WM, the wavelengths to avoid are the zero dispersion wavelengths of the sections of the cable nearest to the end of the cable at which the signals are inserted, because those are the sections in which the optical intensity is highest and in which 4 WM is therefore most severe. When the signals have propagated through several sections of the cable, they have become attenuated, and 4 WM, which is a high order non-linear effect, ceases to be a great problem.